Why do I need to be in the Primary position when passing side roads?
As cars and other vehicles approach side roads, they give a quick glance to see whether or not cars or other motor vehicles are approaching. Positioning yourself in the primary position and acting like a motor vehicle helps you in three ways:
-It allows vehicles emerging from the side road to see you better
-It deters vehicles behind you from overtaking and quickly turning left across your path (the left hook)
-It allows approaching cars wanting to turn right into the side road to see you better

fig.1

How do I move into the correct position?
If you're approaching the junction already in the primary position, great, stay there, if you're in the secondary position and need to move out, glance behind you and if it is safe to do so move out into the primary position as shown by the green line curving away from the orange line in fig.1. You don't need to signal, keep your hands covering the brakes.

What do I do as I'm approaching and passing the side road?
Even though moving to the primary position greatly increases your visibility, you still need to keep your eyes open, be aware of any vehicles turning into or out of the junction that may or may not have seen you. Depending on visibility when emerging from a side road, some motor vehicles may roll slightly over the stop line to see what is coming. You'll see from fig.2 that having moved from the orange line (secondary) to the green line (primary) not only are you more visible, but you will avoid these vehicles that are edging out of the side road.

fig.2

What do I do after the side road?
Assuming that the road is quiet and there are no more side roads or parked cars imminently, you can move back into the secondary position. Don't be moving back and forth all the time, be predictable. If you can see another side road or parked car ahead of you, (as we can in fig.2) you are safer if you stay in the primary position in that gap rather than swinging left and then having to come out again just a few yards further on.

What if it's too busy to be moving out to the primary position at every side road?
On busy main roads it's sometimes not possible or practical to move completely to the primary position. You should still give a quick glance behind and if safe to do so, edge out a little to deter drivers behind from performing a left hook and increase your visibility. Again, you don't need to signal, cover the brakes with your hands and remember to acknowledge any courtesy from drivers. If you can't move out, just be more vigilant of traffic entering and exiting the side road, try to make eye contact and anticipate what other vehicles are going to do before they do it.

fig.3

What are the dangers with parked cars then?
Aside from the more obvious "dooring" where the door of a parked car opens into your path, pedestrians may step from behind a vehicle if crossing the road, and the parked vehicle may set off suddenly. Being in the primary position and at least a doors width away from the car (fig.3) gives you that extra space and time to avoid hazards should one appear, as well as making you more visible.

What if it's too tight for me to leave a doors width?
Be vigilant, try to look through the cars for people either sitting in them or crossing between them. Use your ears as well as eyes to spot cars starting up ready to move off. Be aware of your surroundings, cover the brakes and be ready to act if necessary. If in doubt when passing side roads or parked cars, stop, get off and either assess the situation, or walk your bike past it on the pavement.